This invention relates in general to electro-hydraulic brake systems and in particular to compensation of pressure signals generated by a cluster of pressure sensors upon an electro-hydraulic brake system control unit.
An electro-hydraulic brake system (EHB) combines the advantages of an electric braking system with components of a conventional hydraulic brake system. Thus, an EHB can be considered as an intermediate hybrid system which includes features of both a conventional hydraulic brake system and a brake by wire system (BBW). By utilizing conventional hydraulic brake components, development and conformance costs and times are reduced.
Referring now to FIG. 1, there is shown, generally at 10, a typical EHB. The EHB 10 includes a pedal unit 11 which is hydraulically connected to a hydraulic control unit (HCU) 12. The hydraulic control unit 12 forms an interface between the pedal unit 11 and a pair of conventional hydraulically actuated vehicle front wheel brakes 13 and a pair of conventional hydraulically actuated vehicle rear wheel brakes 14.
The pedal unit 11 includes a tandem master cylinder 15 which is supplied with brake fluid from a master cylinder reservoir 16. The master cylinder 15 is connected by a conventional mechanical linkage to a vehicle brake pedal 17. The brake pedal 17 also is coupled to a displacement transducer 18 which generates an electrical signal having an amplitude which is proportional to brake pedal travel. One chamber of the master cylinder 15 is connected by a first hydraulic brake line 19 while the other chamber of the master cylinder 15 is connected to a second hydraulic brake line 20. The pedal unit 11 also includes a normally closed valve 21 which connects the first brake hydraulic brake line 20 circuit to a pedal travel simulator 22. The pedal travel simulator 22 is an electro-hydraulic device which is operative during operation of the EHB 10 to provide brake pedal resistance and force as feedback to the vehicle operator.
The HCU 12 includes a first normally open isolation valve 23 which is connected between the first hydraulic brake line 19 and one of the front wheel brakes 13 and a second normally open isolation valve 24 which is connected between the second hydraulic brake line 20 and the other of the front wheel brakes 13. Each of the front wheel brakes 13 is connected through an isolator piston 25 to a pair of proportional control valves 26 whose purpose will be explained below. The second valve of each pair of proportional control valves 26 is connected to a corresponding rear wheel brake 14. The isolator pistons 25 hydraulically isolate the front wheel brakes 13 from the rear wheel brakes 14. As shown in FIG. 1, the front wheel brakes 13 are hydraulically connected through a first balance valve 27. Similarly, the rear wheel brakes 14 are hydraulically connected through a second balance valve 28.
The HCU 12 further includes a motor driven pump 35 as a source of pressurized brake fluid for actuation of the wheel brakes 13 and 14. The pump 35 has an intake port which draws brake fluid through a hydraulic line 36 from the master cylinder reservoir 16. The pump 35 also has a discharge port which is connected through each of the proportional control valves 26 to a corresponding front or rear wheel brake 13 or 14. Each of the proportional control valves 26 includes a discharge port which is connected though a hydraulic discharge line 38 to the master cylinder reservoir 16. The discharge port of the pump 35 also is connected through a relief valve 39 to a high pressure accumulator 40.
A plurality of pressure sensors are included in the EHB 10. The pressure applied to the HCU 12 by the master cylinder 15 is monitored by a brake actuation pressure sensor 45 which is illustrated in FIG. 1 as being mounted in the first hydraulic brake line 19 between the master cylinder 15 and the first isolation valve 23. Alternately, the brake actuation pressure sensor 45 can be mounted in the second hydraulic brake line 20 between the master cylinder 15 and the second isolation valve 24 (not shown). The brake actuation pressure sensor 45 is rated to measure relatively low pressures which are on an order of magnitude of 60 bar (900 psi). A wheel brake pressure sensor 47 is included in each hydraulic line connecting each proportional control valve 26 to the associated wheel brake. The wheel brake pressure sensors 47 monitor the pressure being applied to the associated wheel brake and are rated to measure relatively high brake actuation pressures which are on an order of magnitude of 200 bar (3,000 psi). An accumulator pressure sensor 48 is connected to the high pressure accumulator 40 and monitors the output pressure of the accumulator 40. When the pump pressure exceeds the accumulator pressure or when the relief valve 39 is open, the accumulator pressure sensor 48 measures the pump output pressure. The accumulator pressure sensor 48 is also rated to measure relatively high pressures which are on an order of magnitude of 200 bar (3,000 psi).
The solenoid valves and pressure sensors are electrically connected to a microprocessor (not shown) which is included in an Electronic Control Module (ECU) (not shown). The ECU can either be mounted upon the HCU 12 or located remotely from the HCU 12. The ECU microprocessor is programmed with appropriate software to monitor the output signals from the pressure sensors 45, 47 and 48 and the brake pedal transducer 18. The microprocessor is responsive to the sensed pressures and displacement of the brake pedal transducer 18 to energize the pump 35 and to selectively actuate the proportional control valves 26 to supply pressurized hydraulic fluid for actuation of the wheel brakes 13 and 14.
The operation to the EHB 10 will now be described. During vehicle operation, the microprocessor associated with the HCU 12 continuously receives electrical signals from the brake pedal transducer 18 and the pressure sensors 45, 47 and 48. The microprocessor monitors the condition of the brake pedal transducer 18 and the pressure signals from the brake actuation pressure sensors 45 for potential brake applications. When the vehicle brake pedal 17 is depressed, the brake pedal displacement transducer 18 generates a displacement signal. Simultaneously, the brake actuation pressure sensor 45 generates a signal which is proportional to the force applied to the brake pedal 17. The microprocessor is operative to combine the displacement and force signals into a brake command signal. The microprocessor software is responsive to the brake command signal to actuate the pump motor and close the isolation valves 23 and 24 to separate the master cylinder 15 from the wheel brakes 13 and 14. The microprocessor then selectively operates the proportional control valves 26 in the HCU 12 unit to cyclically relieve and reapply hydraulic pressure to the wheel brakes 13 and 14. The hydraulic pressure applied to the wheel brakes is adjusted by the operation of the proportional control valves 26 to produce adequate brake torque to decelerate the vehicle in accordance with the brake command signal generated by the vehicle operator.
If the EHB 10 should fail, the isolation valves 23 and 24 return to their normally open positions to provide unassisted push though braking by allowing direct hydraulic communication between the master cylinder 15 and the front wheel brakes 13.
This invention relates to a method and apparatus for compensation of pressure signals generated by a cluster of pressure sensors upon an electro-hydraulic brake system control unit.
It is known to include multiple pressure sensors in an electro-hydraulic brake unit. However, each sensor and associated bridge circuitry requires a compensation circuit to correct for offset voltages. Accordingly, it is known to provide a compensation circuit for each sensor. Additionally, associated signal conditioning circuitry may include a differential amplifier which requires compensation for offset voltage and/or linearity. However, the number of compensation circuits increases the complexity of the circuitry and requires a significant number of electrical connectors between the sensors and the electro-hydraulic brake unit microprocessor. Accordingly, it would be desirable to provide a simpler method for compensation of the pressure sensors.
The present invention contemplates a hydraulic control valve that includes a control valve body having a plurality of passageways formed therein. The passageways are adapted to be connected to a hydraulic control system. A plurality of pressure sensors are carried by the control valve body with each of the pressure sensors communicating with a selected one of the passageways. The sensors are operative to generate a pressure signal voltage that is a function of a fluid pressure in the selected passageway. The control valve also has a signal conditioning circuit carried by the control valve body. The signal conditioning circuit is operative to sequentially sample the pressure signal voltages and to generate an analog multiplexed signal that includes the pressure signal voltage samples.
It is further contemplated that the pressure sensors include a bridge circuit. Each of the bridge circuits generates a first output voltage and a second output voltage with the difference between the first and second output voltages being proportional to the fluid pressure. The signal conditioning circuit also includes a first multiplexer having a plurality of input terminals with a selected input terminal connected to the first bridge output voltage of each sensor bridge circuit. Similarly, a second multiplexier having a pluraltiy of input terminals has a selected input terminal connected to the second output voltage of each bridge circuit. The first multiplexer is operative to generate a first multiplexed output voltage at a first output terminal while the second multiplexer is operative to generate a second multiplexed output voltage at a second output terminal. The first multiplexer output terminal is connected to a first input terminal of a differential amplifier and the second multiplexer output terminal is connected to a second input terminal of the differential amplifier. The differential amplifier has an output terminal connected to an input port of a microprocessor. The differential amplifier is operational to generate a multiplexed pressure voltage signal that includes the difference of each of the bridge first and second voltage signals and to apply the multiplexed pressure voltage signal to the microprocessor input port.
The signal conditioning circuit also includes a first digital resistor connected across a voltage supply. The first digital resistor has an output tap terminal connected to an input terminal of the differential amplifier and a control terminal connected to the microprocessor. The microprocessor is operative to select a value for the first digital resistor that corresponds to a particular sensor bridge circuit. The first digital resistor causes a voltage to be applied to said differential amplifier input port to compensate for an offset voltage generated by the sensor bridge circuit and/or the differential amplifier.
The hydraulic control valve also can include a temperature sensor mounted upon the valve body and connected to the microprocessor. The temperature sensor is operative to generate a temperature signal that is proportional to the temperature of the hydraulic fluid. The microprocessor is operative to select a compensating digital resistor value that is a function of both the bridge circuit being sensed and the fluid temperature.
The signal conditioning circuit also can include a second digital resistor connected between the output terminal of the differential amplifier and an input terminal thereof. The second digital resistor has an output tap terminal connected to the same differential amplifier input terminal and a control terminal connected to the microprocessor. The microprocessor is operative to select a value for the second digital resistor that corresponds to a particular sensor bridge circuit to provide gain compensation for the differential amplifier.
Alternately, the signal conditioning circuit can include a digital to analog converter having an output terminal connected to an input terminal of the differential amplifier and an input terminal connected to the microprocessor. The microprocessor is operative to select an input value for the digital to analog converter that corresponds to a particular sensor bridge circuit. As a result, the digital to analog converter will cause a voltage to be applied to the differential amplifier input port to compensate for an offset voltage generated by the sensor bridge circuit and/or the differential amplifier.
As another alternate, the signal conditioning circuit can include a third multiplexer having a plurality of input terminals. Each of the multiplexer input terminals is connected to a charged capacitor. The third multiplexer also has an output terminal connected to an input terminal of the differential amplifier and a control terminal connected to the microprocessor. The microprocessor is operative to cause the third microprocessor to selectively connect one of the charged capacitors that corresponds to a particular sensor bridge circuit to the differential amplifier input terminal. As a result, the charged capacitor will apply a voltage to the differential amplifier input port to compensate for an offset voltage. It is contemplated that the microprocessor sequentially connects the capacitors to the differential amplifier input terminal to provide compensation for each of the sensor bridge circuits and/or the differential amplifier. While one of the capacitors is connected to the differential amplifier, the other capacitors are recharged. The capacitors are recharged with a Pulse Width Modulated signal having a variable duty cycle. The duty cycle is selected to charge the corresponding capacitor to a desired voltage level.
The invention also contemplates a method for compensating a signal conditioning circuit including providing a signal conditioning circuit connected to a plurality of bridge circuits. The signal conditioning circuit includes a differential amplifier having a compensating component connected to an input terminal thereof. The signal conditioning circuit is operative to sequentially sample the output voltages of the individual bridge circuits. A value for the compensating component is selected that corresponds to the individual bridge circuit being sampled. Then the compensating component is caused to apply a compensating voltage to the differential amplifier input terminal that is a function of the selected value whereby the differential amplifier offset is compensated.
Additionally, the signal conditioning circuit also can include a temperature sensor with the method including sensing the temperature and selecting a value for the compensating component that is a function of both the individual bridge circuit being sampled and the temperature.
Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings.